Production of vitreous silica in a rotary kiln



United States Patent 3,326,704 PRODUCTION OF VITREOUS SILICA IN A ROTARY KILN Robert K. Scott, Pittsburgh, Earl Leatham, Wexford, and

Edward Einstein, Bethel Park, Pa., assignors to Harbison-Walker Refractories Company, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Filed Sept. 3, 1965, Ser. No. 485,111 The portion of the term of the patent subsequent to Apr. 18, 1984, has been disclaimed 6 Claims. (Cl. 106-65) The present invention relates to the production of vitreous silica, particularly its production in a rotary kiln and at temperatures below the true melting point of silica.

Vitreous silica is a highly desirable refractory material because of its great thermal shock resistance, which is attributable to its very low rate of reversible thermal expansion.

The phase transformations of silica are numerous but stated in their simplest form; the major modifications (allotropic phases) are quartz, tridymite, cristobalite, and fused silica, which is non-crystalline silica or silica glass normally produced by fusion and quenching. There are many aspects governing the transformation of one to the other, but in highly simplified form, the equilibrium ranges are as follows:

Range of stability Quartz Room temperature to 870 C. (1598 F.). Tridymite 870 C. to 1470 C. (2678 F.). Cristobalite 1470? C. to 1725 C. (3140 F.). Cristobalite melts to form fused silica at 3140 F.

Each of the major crystalline phases, quartz, tridymite, and cristobalite, has two or more forms with their own stability relationships, but these need not be discussed here. The most complete literature reference to this subject is the book, The Properties of Silica, by Robert B. Sosman, published as an American Chemical Society monograph by the Chemical Catalog Company, Inc. (1927).

To illustrate the complexity of the phase relationships existing between the various forms of silica, one example might be given: On heating quartz, which is the form of silica most abundant in nature, its inversion does not necessarily proceed quartz tridymite cristobalite; it is far more usual for tridymite to be at least temporarily by-passed and for the quartz to be converted, first to cristobalite and thereafter (if at all) in some degree to tridymite. This reluctance to form tridymite in its equilibrium range is related to the fact that the presence of a flux is required, to bring about its direct formation from quartz. I

We have observed, through mineralogic studies involving the X-ray, that when quartz is heated above its equilibrium range (1598 F.) an amorphous or disorganized state results, beginning at about 1250 C. (2282 F.), and that observable cristobalite does not develop until a somewhat higher temperature. This observation will later be commented on more fully, in connection with our discovery of practical means for bringing about the formation of vitreous silica at low tempertures.

In theory, each of the three major crystalline forms of silica could have its own melting point; but this has chiefly been of interest to theoreticians since, in fact, the normal heating of quartz and tridymite results in the formation of cristobalite, which is the form of silica that melts to a liquid, and when cooled form fused silica. In discussing these matters, Dr. Sosman uses such phrases, The melting point of tridymite, if realizable and points out that, While in theory the melting point of quartz is probably below 2678 F., This is not the tem- 3,326,704 Patented June 20, 1967 the high temperture stable form, cristobalite. Because of these relationships observed many times, such fusing of quartz to form fused silica, as has been practiced, has been accomplished at temperatures above 3140 F., the melting point of cristobalite, and with considerable difficulty. Fusing silica is an expensive process since the high temperatures required for melting have dictated, in general, the use of electric furnaces.

We have discovered that it is possible to produce material which is nearly all vitreous silica, having the properties of silica which has been fused, at a temperature lower than the melting point of cristobalite. We have accomplished this by heating quartz materials below the melting point of cristobalite, in a rotary kiln, at fast heating rates and under conditions which indicate we have commercially achieved the direct vitrification of quartz, without going through the cristobalite phase and without true melting. Thus, a means has been found of achieving an end, which has been theoretically possible but which has previously evaded prior workers.

We believe that our success results from our realization that, at temperatures in excess of 1250 C., quartz decomposes at a finite rate, not instantaneously, to the disorganized or amorphous state, the rate being determined by the crystal size and the temperature and, probably, also, by chemicalcomposition, i.e., the nature and amount of impurities. The formation of cristobalite, from the disorganized or amorphous state, also proceeds at a rate dependent on temperature and chemical composition, which rate is somewhat dilferent than the rate of decomposition of quartz. Both reactions appear to increase in rate with increasing temperature between 1250. C:. and 1700 C., the former increasing more rapidly than the latter.

Thus, if we select a suitable raw material and temperature, so that the rate of decomposition of the quartz greatly exceeds the rate of cristobalite formation, and se lect the time of hold at temperature in the proper range, we are able to obtain nearly complete decomposition of quartz, with the formation of only a minor amount of cristobalite. If the largely amorphous mass be then rapidly cooled, the product remains vitreous and is similar in properties to quartz, which has been completely fused and rapidly cooled. The rotary kiln process allows the application of intense heat to all surfaces of the material as it tumbles, so that the formation of amorphous, or vitreous, silica has progressed to a high degree, before a large amount of cristobalite is formed.

The temperature at which the rate of quartz breakdown exceeds the rate of formation of cristobalite with the raw material and temperatures employed is about 2900 F. We have discovered and it is upon this that our invention is largely predicated, that a small addition of alumina or alumina containing material to the surfaces of the silica raw material to be vitrified lowers the temperature (about 2800 F.) at which the rate of quartz breakdown exceeds the rate of formation of cristobalite; The propagation of the cristobalite phase through the rock is thus retarded at the surfaces increasing the yield of vitreous silica.

It is an object of the present invention to provide a vitreous silica aggregate containing up to 10%, by weight, of cristobalite.

Another object of the invention is to produce vitreous silica in a more simple, economical, and eflicient manner than was heretofore possible. I

A further object of the invention is to produce vitreous silica in a rotary kiln at a temperature lower than the melting point of cristobalite.

Other objects of the invention will appear hereinafter.

Briefly, in accordance with one embodiment of the present invention, the novel process includes applying a coating of alumina or alumina containing materials on the silica raw material in an amount between .25 and 1 part per 100 parts of raw material, charging the coated silica raw material to a rotary kiln, heating it continuously and rapidly (i.e. at a rate of at least about 25 F. per minute) to a temperature of about 2900 F. to 3100 F, holding the material thereat for a period of about /2 to 1 /2 hours, then cooling the resulting product to a temperature below about 2000 F. rapidly, preferably, at a rate of about 200 F. per minute and recovering a vitreous silica aggregate.

A preferred raw material, for processing according to the teaching of the invention, is a sedimentary quartzite occurring naturally in massive form, having an average crystallite diameter of about 10 microns and less, preferably between 5 and 10 microns, and containing negligible intercrystalline material. A preferable useable silica raw material contains at least 99% SiO on an oxide basis. Such a silica raw material may be found, for instance, in Arkansas, Illinois, and California and has the following typical chemical analysis:

Percent SiO 99.2 Fe 'O 0.10 A1 0.14 CaiO 0.05 TiO 0.01 MgO 0.03 Alkalies 0.27 Ignition loss 0.20

In practice, the quartzite material is subjected to a light crush, after removal from the mine, to provide lumps averaging about /2" to 2" in diameter. Uniformity of feed sizing is desirable, in order to maintain a uniform rate of flow of the material through the rotary kiln. The crushed rock may be fed to the kiln by any suitable means, such as, a conveyor belt. The rock is sprayed with a clay slurried in water in an amount sufficient to .provide about .25 to 1 part alumina per 100 parts of silica rock. A suitable clay material is crude Alabama kaolin analyzing approximately 52% SiO 45% A1 0 1.6% TiO 1.3% Fe O and the balance CaO, MgO, and alkaline. Other suitable clays are kaolinitic, ball and plastic clays. Also crude bauxite and pure alumina, such as hydrated alumina, could be used.

The material is subjected to the burning zone of the kiln, which is maintained at a temperature of between 2900 to 3100 F. Although the temperature at which cristobalite melts was given previously to be about 3140" F, vitreous silica will form at about 2900 F. after a period of about /2 hour, when the material is subjected to this temperature without substantial preheating. The temperature of the charge material must be kept below the melting point of cristobalite, and preferably below 3-100 F., since above this temperature, the vitreous material being formed tends to become sticky and adheres to the walls of the kiln, so as to make it virtually inoperative. Also, it is important that the rock does not have a residence time at these temperatures exceeding about 1 /2 hours, since some of the vitreous silica may devitrify and form cristobalite. After the prescribed period of residence of the material in the kiln, it is discharged from the kiln and rapidly cooled to a temperature below about 2000 F. in a period of the order of 5 minutes. Rapid cooling may be accomplished by subjecting the discharged material to a blast of cold air or by quenching the material in Water. The latter method has been preferred since the aggregate is much easier to handle.

Batches, processed in accordance with the above process, have been analyzed after discharge and cooling from the kiln and have been found to consist primarily of vitreous silica aggregate, with only about 10%, by weight, cristobalite present and as little as 2%.

The following example illustrates more clearly the teachings of the present invention.

A sedimentary quartzite rock, having a typical chemical analysis mentioned previously, was shattered by impact blows to provide lumps ranging in diameter from /2" to 2". The rock was sprayed with about 1.5% of dust collector clay slurry to provide a coating thereon. This material was fed to a rotary kiln having a reducing atmosphere, containing carbon monoxide, at a rate of about 8 tons per hour. The rate of rotation of the kiln was about 1 /2 revolutions per minute. The gas temperature at the feed end of the kiln was found to be about 600 F., and the burning zone of the kiln was maintained at about 3020 F. The material was subjected to the burning zone temperatures for about 1 /2 hours. After residence, the material was immediately discharged and quenched into a tank filled with water. X-ray analysis of the cooled aggregate indicated that vitreous silica was present in an amount in excess of 92%.

It is intended that the foregoing description be construed as illustrative and not in limitation of the invention.

The present invention is an improvement in application Ser. No. 392,320 filed Aug. 26, 1964 and assigned to the assignees of the present invention.

Having thus described the invention in detail and with suflicient particularity as to enable those skilled in the art to practice it, What is desired to have protected by Letters Patent is set forth in the following claims.

We claim:

1. A process for continuously producing a vitreous silica aggregate containing no more than about 10% cristobalite comprising applying a coating selected from the group consisting of alumina and alumina containing materials to a crystalline silica raw material occurring naturally in massive form having an average crystallite diameter of about 10 microns and less and containing negligible intercrystalline material in an amount sufficient to provide between about 0.25 and 1 part alumina per parts of material, continuously feeding the coated material to a rotary kiln, maintaining the hot zone temperature in the kiln above the equilibrium melting point of quartz but below the equilibrium melting point of silica, allowing the material to tumble downwardly through the kiln so as to allow the application of intense heat to all surfaces of the material, holding the material in said hot zone for a period not exceeding about 1 /2 hours but sufficient to decompose the crystalline structure and provide a predominantly amorphous state in said material, then rapidly cooling the resulting product and recovering a vitreous silica aggregate containing no more than about 10% cristobalite.

2. The process of claim 1 in which the hot zone temperature of the rotary kiln is between about 2900 and 3100 F.

3. The process of claim 1 in which the material is held in said hot zone for a minimum period of about /2 hour.

4. The process of claim 1 in which the silica material is cooled to a temperature below about 2000 F. at a rate of about 200 per minute.

5. The process of claim 1 in which the silica raw material is rapidly cooled by quenching in water.

6. The method of claim 1 in which the silica raw material contains at least 99% SiO on an oxide basis.

References Cited Martinez, Comptes Rendus, vol. 223, pages 657659 (1946).

TOBIAS E. LEVOW, Primary Examiner.

HELEN M. McCARTHY, Examiner.

J. E. POER, Assistant Examiner. 

1. A PROCESS FOR CONTINUOUSLY PRODUCING A VITREOUS SILICA AGGREGATE CONTAINING NO MORE THAN ABOUT 10% CRISTOBALITE COMPRISING APPLYING A COATING SELECTED FROM THE GROUP CONSISTING OF ALUMINA AND ALUMINA CONTAINING MATERIALS TO A CRYSTALLINE SILICA RAW MATERIAL OCCURRING NATURALLY IN MASSIVE FORM HAVING AN AVERAGE CRYSTALLITE DIAMETER OF ABOUT 10 MICRONS AND LESS AND CONTAINING NEGLIGIBLE INTERCRYSTALLINE MATERIAL IN AN AMOUNT SUFFICIENT TO PROVIDE BETWEEN ABOUT 0.25 AND 1 PART ALUMINA PER 100 PARTS OF MATERIAL, CONTINUOUSLY FEEDING THE COATED MATERIAL TO A ROTARY KILN, MAINTAINING THE HOT ZONE TEMPERATURE IN THE KILN ABOVE THE EQUILIBRIUM MELTING POINT OF QUARTZ BUT BELOW THE EQUILIBRIUM MELTING POINT OF SILICA, ALLOWING THE MATERIAL TO TUMBLE DOWNWARDLY THROUGH THE KILN SO AS TO ALLOW THE APPLICATION OF INTENSE HEAT TO ALL SURFACES OF THE MATERIAL, HOLDING THE MATERIAL IN SAID HOT ZONE FOR A PERIOD NOT EXCEEDING ABOUT 1 1/2 HOURS BUT SUFFICIENT TO DECOMPOSE THE CRYSTALLINE STRUCTURE AND PROVIDE A PREDOMINANTLY AMORPHOUS STATE IN SAID MATERIAL, THEN RAPIDLY COOLING THE RESULTING PRODUCT AND RECOVERING A VITREOUS SILICA AGGREGATE CONTAINING NO MORE THAN ABOUT 10% CRISTOBALITE. 